1. Field of the Invention
The present invention relates to a method of producing a coupling, preferably a coupling for oil country tubular goods (hereinafter referred to as an O.C.T.G.) having a special screw thread.
2. Description of the Related Art
Very deep oil wells are being drilled lately. Accordingly, the steel pipes and couplings used for such wells need higher strength.
Various couplings for tubing or casings have been developed in strength and dementional accuracy, for example, couplings having special screw threads, such as a so-called VAM coupling or a FOX coupling, as disclosed in Japanese Patent Laid-open No. 60-69385 (and as shown in FIG. 5).
As shown in FIG. 5, such a threaded coupling 1 has metal seals 1a provided at an Internal Bore Dimension 1d thereof, and special screw threads 1c tapered from the Internal Bore Dimension 1d toward the ends 1b, thereby achieving good sealing.
According to the conventional method, such special pipe couplings are produced by a process as illustrated in FIG. 7. A plain pipe 8 for a coupling 1, as shown in FIG. 6, has an outside diameter Ds slightly greater than the outside diameter Dc required for the coupling 1, and has a wall thickness ts slightly greater than the maximum wall thickness tmax required for the coupling 1. After the entire plain pipe 8 is heat-treated, that is, quenched and tempered, it is cut into pipe pieces having a predetermined length Ls. The outside surfaces of the pieces are finished on a lathe turning machine, and their inside surfaces are also machined and then threaded on a lathe turning machine. Finally, the pieces are coated or plated.
The above conventional method is described in Japanese Patent Laid-open No. 56-24277, column 2, line 17-20, translated as follows. "Because such a known coupling is formed substantially by turning the inside surface of a coupling material having a wall thickness greater than the final wall thickness by a thickness of a shoulder portion 2, the process takes a long time, and produces a considerably large amount of material waste."
Because the method must use a plain pipe having a wall thickness equal to or greater than the final maximum wall thickness of the coupling, the production yield results in very low amount. Further, because a significantly large amount of material must be machined, the working efficiency of the turning process becomes significantly low.